Induction of Ig somatic hypermutation and class switching in a human monoclonal IgM+ IgD+ B cell line in vitro: definition of the requirements and modalities of hypermutation.

Partly because of the lack of a suitable in vitro model, the trigger(s) and the mechanism(s) of somatic hypermutation in Ig genes are largely unknown. We have analyzed the hypermutation potential of human CL-01 lymphocytes, our monoclonal model of germinal center B cell differentiation. These cells are surface IgM+ IgD+ and, in the absence of T cells, switch to IgG, IgA, and IgE in response to CD40:CD40 ligand engagement and exposure to appropriate cytokines. We show here that CL-01 cells can be induced to effectively mutate the expressed VHDJH-C mu, VHDJH-C delta, VHDJH-C gamma, VHDJH-C alpha, VHDJH-C epsilon, and V lambda J lambda-C lambda transcripts before and after Ig class switching in a stepwise fashion. In these cells, induction of somatic mutations required cross-linking of the surface receptor for Ag and T cell contact through CD40:CD40 ligand and CD80: CD28 coengagement. The induced mutations showed intrinsic features of Ig V(D)J hypermutation in that they comprised 110 base substitutions (97 in the heavy chain and 13 in the lambda-chain) and only 2 deletions and targeted V(D)J, virtually sparing CH and C lambda. These mutations were more abundant in secondary VHDJH-C gamma than primary VHDJH-C mu transcripts and in V(D)J-C than V lambda J lambda-C lambda transcripts. These mutations were also associated with coding DNA strand polarity and showed an overall rate of 2.42 x 10(-4) base changes/cell division in VHDJH-CH transcripts. Transitions were favored over transversions, and G nucleotides were preferentially targeted, mainly in the context of AG dinucleotides. Thus, in CL-01 cells, Ig somatic hypermutation is readily inducible by stimuli different from those required for class switching and displays discrete base substitution modalities.